But the cost of Amazon Prime have to be factored in as well

Only if you weren't already paying for Prime. If you were, then it's irrelevant as this usage adds no marginal cost.

"If you already pay for shipping then shipping adds no marginal cost."

	▲	OkayPhysicist 10 hours ago \| parent [-]
		well, yeah. That's what the phrase "marginal cost" means: if it costs $x to do something N times, and $x + $y to do something N+1 times, $y is the marginal cost.

No, for anyone considering doing this it does not have to be factored in as it’s a sunk cost.